1. Field of the Invention
The present invention relates to a volume holographic memory and an optical information recording and reproducing apparatus using the volume holographic memory.
2. Description of the Related Art
Conventionally, a holographic memory system is known as a digital recording system using the principle of holography. The holographic memory system records digital data on a memory medium made of a photorefractive crystalline such as lithium niobate (LiNbO.sub.3) or the like, and reproduces the data from the same. The photorefractive effect is a phenomenon in such that electric charges generated by photo-excitation move within a crystal thereby to form a spatial electric field distribution, which combines with a primary electro-optical effect i.e., Pockels effect to change a refractive index distribution in the crystal. In a ferroelectric crystal or the like exhibiting the photo-refractive effect, its change of the refractive index is responsive even to a fine optical input pattern of 1,000 lines or more per one millimeter, and this effective action is generated at a response speed on the order of microseconds to seconds in real time, though the response speed varies depending on kinds of materials. Therefore, a variety of applications for such crystals has been studied as a real time hologram medium which does not require any developing. The holographic memory system is capable of recording and reproducing data on a two-dimensional plane page unit, and also performing a multiple recording with use of a plurality of the page units. The volume holographic memory is designed to enable three-dimensional recording with a crystal medium being of a three-dimensional shape such as a rectangular parallelepiped or the like. In the volume holographic memory, which is one type of Fourier transform holograms, data is recorded at every two-dimensional image page unit in a dispersed manner within a three-dimensional space of the volume holographic memory. In the following, the outline of the holographic memory system will be described with reference to FIG. 1.
Referring to FIG. 1, an encoder 25 translates digital data to be recorded in a volume holographic memory 1 into a dot pattern image consisting of light and dark spots arranged in a plane, and rearranges the image in a data arrangement, for example, a data array of 480 pixels in the vertical direction and 640 pixels in the horizontal direction to generate a unit page sequence data. The unitary page sequence data is supplied to a spatial light modulator (SLM) 12 including a panel of a transmission type Thin Film Transistor (TFT) liquid crystal display (hereinafter also called simply as "LCD").
The spatial light modulator 12 has a modulation unit for performing a modulation processing of 480 pixels in a line and 640 pixels in a row which corresponds to one unit page, and optically modulates a light beam into an on/off signal of spatial light in accordance with the unit page sequence data from the encoder 25, and guides the modulated light beam, i.e., signal light beam to a lens 13. More specifically, the spatial light modulator 12 passes therethrough the light beam in response to a logical value "1" of the unit page sequence data, which is an electric signal, and shuts off the light beam in response to a logical value "0" thereby to accomplish the electro-optical conversion in accordance with the contents of respective bits in the unit page data. Accordingly, the signal light beam including the unit page sequence is generated by modulation of the light beam.
The signal light beam is incident upon the volume holographic memory 1 through the lens 13. In addition to the signal light beam, a reference light beam is incident upon the volume holographic memory 1 at an angle .beta. (hereinafter, referred to as "incident angle .beta.") relative to a predetermined baseline perpendicular to an optical path of the signal light beam.
Both the signal light beam and the reference light beams interfere with each other within the volume holographic memory 1, and the resulting interference fringes are stored as a refractive index grating within the volume holographic memory 1, whereby recording of data is effected. Also, when the volume holographic memory 1 is irradiated multiple times with the reference light beam at different incident angles .beta. to record a plurality of two-dimensional plane data in an angle multiplexing form, a recording of three-dimensional data can be accomplished.
When reproducing the recorded data from the volume holographic memory 1, only the reference light beam is introduced into the volume holographic memory 1 at the same incident angle .beta. as at the time of recording toward the center of a region in which the signal and reference light beams intersect with each other. In other words, the reproducing of the recorded data is different from the recording of the data in that the signal light beam is not introduced into the volume holographic memory 1. Therefore, the volume holographic memory 1 diffracts the reference light beam at the intersection of the refractive index grating caused by interference fringes. The diffracted light from the refractive index grating recorded in the volume holographic memory 1 is guided through a lens 21 to a photodetector such as a Charge Coupled Device (CCD) 22 on which a light and dark pattern image i.e., an image of the data arrangement is reproduced. The CCD 22 converts the received image into variations in intensity of an electric signal to output to a decoder 26 an analog electric signal having a level corresponding to a distribution of brightness in the incident image. The decoder 26 compares the analog electric signal with a predetermined amplitude i.e., a slice level to reproduce data consisting of the corresponding "1" and "0".
Since the volume holographic memory records two-dimensional plane data sequences as described above, angle multiplexing recording can be performed by changing the incident angle .beta. of the reference light beam. Specifically, a plurality of two-dimensional planes, i.e., the recorded units, can be defined within the volume holographic memory by changing the incident angle .beta. of the reference light beam. Consequently, three-dimensional recording can be carried out. Examples of angle multiplexing recording are described in Japanese Unexamined Patent Publications Kokai Nos. H5-142979 and H10-97174.